Cocconeis distans Gregory

13. Cocconeis distans Gregory (Figs 12–19)

Basionym: Gregory 1855, p. 39, pl. 4, fig. 9 (incorrect); Gregory 1857, p. 490, pl. 9, fig. 23 (correct) (De Stefano et al. 2006).

References: Sar et al. 2003, p. 86, figs 16–21, as Cocconeis guttata Hustedt & Aleem; De Stefano et al. 2006, p. 440, figs 1–7, 15–32.

Description: Valves elliptical, often rhombic in large forms, 13–31 μm long, 8–23 μm wide. SV: sternum linear, central area not developed. Transapical striae radiate towards the apices, 6–9 in 10 μm, areolae on striae, coarsely round to subquadrangular and included inflated costae around each areola, 5–9 in 10 μm of stria. A row of areolae distinct around the margins of valve, on the other hand, among some large forms, two or three small areoale regularly arranged at the terminal ends of each stria in the valve margins (Fig. 13). RV: raphe straight, sternum narrow. Central area not developed. A narrow, hyaline area in the margin borders between valve and mantle. Transapical striae radiate towards the apices, 15–20 in 10 μm, with 12–19 areolae in 10 μm of stria.

Remarks: Cocconeis distans and C. guttata are occasionally confused due to their similar morphology of valves, and until recently, taxonomic confusion still exists. A close examination of the literature describing the two species found that, before comparing the two taxa, there were two morphotypes in the C. distans, small form and very large form in terms of dimensions of valves. As a result of referring to the literature describing small forms (Gregory 1855, Sar et al. 2003, De Stephano et a. 2006, Riaux-Gobin et al. 2011) and large forms (Cleve 1895, Peragallo and Peragallo 1897-1908, Hustedt 1933, Hendey 1964, Riaux-Gobin et al. 2014a), these two morphotypes are clearly distinguished in valve length (18–31: 28–60 μm) and valve width (10–20: 17–40 μm), density of stria (7–11: 4–5 rows in 10 μm), and density of areolae on a stria (4–8: 12–14 in 10 μm of SV). Some morphological differences in length or stria densities may be due to the presence of morphological variations. Genetic analyses will precise if it is the same species or varieties.

Meanwhile, Hustedt (1933) described C. granulifera Greville as a synonym for C. distans while describing C. distans, and until recently the two species are still considered as a heterotypic synonym. However, the synonym of C. granulifera seems to correspond to the large C. distans.

On the other hand, both large and small form of C. distans are clearly distinguished by the difference in the density of the striae on the valve and areolae on the stria. Local taxonomic identity and dimensions of the species are adopted from three references, Sar et al. (2003), De Stefano et al. (2006), and Riaux-Gobin et al. (2011).

This taxon is reported from the Patagonian coast of Argentina (Sar et al. 2003) and is distributed throughout the Mediterranean Sea (De Stefano et al. 2006), and common in coral sediments of Réunion and Rodrigues in the western Indian Ocean (Riaux-Gobin et al. 2011). It occurred infrequently as benthic diatoms in sand bottoms and salt marshes along the Seogwipo coast of Jeju Island. Local assemblages of the species can be divided into two categories, short and long, based on 23 μm in length.